Bug 15601

After commit 905e224849fbbed1719e0add231b00e2d570b3b4 (image_qa: fix error handling), any unexpected exceptions in do_image_qa() would result in a variable being set, but never used, effectively hiding the error.

For Styhead, my recommendation is to just remove the catch of Exception and just let any unexpected exceptions pass on up.

For master I instead recommend that we remove the use of the ImageQAFailed exception (and all exception handling in do_image_qa), and instead rely on the called functions to call oe.qa.handle_error() themselves. This matches what do_package_qa() does.